It's widely regarded as one of the most inhospitable places in the solar system - and yet this hostile world might shed light on the prospect of finding life elsewhere in the universe.

Venus, the brightest celestial object after the sun and moon, frequently appears in the twilight sky as the Evening or Morning Star (Hesperus and Lucifer to the ancient Greeks and Romans) – even though it is a planet and not a star.

The orb is also widely regarded as Earth's "evil twin". Our closest planetary neighbour is considered a twin because of its similar composition and size – with an equatorial diameter of 12,104 kilometres, Venus is almost as big as Earth.

The "evil" connotation is from the hellish atmosphere: it has thick swirling clouds of concentrated sulphuric acid, a crushingly dense atmosphere and temperatures high enough to melt lead.

Needless to say, Venus is one of the least hospitable places in the solar system, which is why its otherworldly surface has remained largely shrouded in mystery.

But not for much longer. Endowed with state-of-the-art instrumentation for peering through the opaque atmosphere of the second planet from the sun, several missions are on the way there.

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First, Akatsuki, a Japanese mission will attempt to start orbiting Venus in December. "It will likely produce promising data and results," says Lori Glaze, the deputy director of the Solar System Exploration Division at NASA's Goddard Space Flight Centre and chair of the Venus Exploration Analysis Group.

"There are several Venus mission concepts under consideration by NASA through the Discovery program that have a chance of being selected for future funded development – and possibly flight," Dr Glaze explains.

The European Space Agency, meanwhile, is gearing up to launch the BepiColombo space probe that will fly by Venus on its way to the innermost planet, Mercury. During the flyby, the robotic craft will relay back high-resolution images.

Yet another mission, called VAMP, which is only at the ideas stage, would – if it gets to fly – attempt to send an inflatable, propeller-powered aircraft through the upper layers of the dense Venusian atmosphere.

Other worlds

So why the intense interest in Venus? "In searching for habitable worlds, we seek planets at the right distance from their star to have liquid water," says Swinburne University astrophysicist Alan Duffy. "This means they should reside in a zone that's just right – in other words, not too close and hot as the oceans would boil but not too far and cold as the oceans would freeze."

This region, of potential habitability, is sometimes known as the Goldilocks zone.

"As we find more and more exoplanets, it seems increasingly likely that there will be more exoVenuses than exoEarths."

Lori Glaze, NASA

Venus, Dr Duffy reminds, is the right size and distance from our sun to tick the habitability box. "But due to an enormously thick atmosphere that traps heat in a runaway greenhouse effect, there are few planets more hostile to life."

The Goldilocks zone was a good starting point for scientists hunting for potentially habitable worlds. "But, as this would also include Venus, it can't be the whole story," Dr Duffy explains.

After finding thousands of alien worlds, and with new telescopes coming online that reveal more about these planets, scientists can be more selective.

"New telescopes, such as ALMA, GMT and the SKA, can tell us about the atmospheres of exoplanets​," he notes. "So, we can separate Venusian worlds from cooler, more Earth-like ones that are in habitable zones."

Thinking the unthinkable

It's often taken for granted that Venus is devoid of life. Yet this may not be true.

"It is unlikely that there is life on the surface of Venus," says another Swinburne astrophysicist, Kurt Liffman. After the surface became uninhabitable, life, being as adaptable as it is, could have shifted into the upper atmosphere – where temperatures and pressures are more Earth-like.

This is not necessarily as implausible as it sounds: bacteria, fungi and plankton – the small drifting plants and animals in seas and lakes – live in some clouds on Earth.

Although the Venusian atmosphere is generally boiling hot and corrosive, this is not the case at all altitudes. In fact, between 50 and 60 kilometres up, the temperatures and pressures are roughly similar to those at sea level on Earth.

"The temperature and pressure at an altitude of 50 kilometres are 75 degrees and 0.97 bar," says Dr Glaze. "At 55 kilometres, it is 29 degrees and 0.5 bar. So, pressures are like sea-level at 50 kilometres and temperatures are more like sea level at 55 kilometres."

Scientists have even found traces of nitrogen, carbon and oxygen – the sort of elements needed for Earthly life. And there's lightning, too.

"Life is astoundingly creative and resourceful - with examples on Earth of life, termed extremophiles, in seemingly impossibly hostile environments," Dr Duffy says. "These grow in soils as acidic as battery acid, thriving in boiling water or crushed under immense pressure hundreds of times greater than we have with our atmosphere."

There is already tentative evidence suggesting biota might exist in the upper atmosphere of Venus. "Part of this is the presence of chemicals or molecules in unexpectedly high abundances – at least compared to what would be expected from standard chemical equilibrium thermodynamics," Dr Liffman says.

To get a clearer idea, astronomers must conduct detailed analyses of the atmosphere of extrasolar planets under examination, Dr Liffman explains. "These may provide clues to the existence of life there."

Not all scientists are this optimistic. "Even if Venus did have life at some time in its past, any evidence would have been destroyed by the extreme climate conditions that exist today," says Dr Glaze.

"Although some people have suggested that microbes might live in the upper cloud layers of Venus – made of sulphuric acid droplets – where temperatures and pressures are more similar to Earth's, it seems unlikely that microbes could have originated in that environment."

So far, she adds, NASA researchers have not noticed anything in the Venus clouds that could be related to "life", at least as we know it.

It is critical, Dr Glaze explains, that scientists understand the Venusian atmosphere so that they can better interpret any exoVenus planets they find. "As we find more and more exoplanets, it seems increasingly likely that there will be more exoVenuses than exoEarths," she points out.

The large mass and density of Venus' carbon dioxide-rich atmosphere drives the very high surface temperatures and pressures. Liquid water, one of the ingredients believed to be key to life, cannot exist under these conditions.

In fact, compared to Earth, "Venus is extremely 'dry'," Dr Glaze notes. "In order to single out those planets that may host life, we need to be able to recognise those that can't."

The best way to find out if there is life on Venus is to go there, says space engineer Jason Held, the director of Saber Astronautics in Sydney.

"Some would argue that the Venusian upper atmosphere offers the highest chance of finding extremophiles," explains Dr Held, who spoke at the recent Amplify Festival in Sydney about the future of space research in Australia. "Among NASA circles, this makes Venus a destination of importance – in competition with Europa, and sometimes even Mars, for research."

Venus in perspective

The reason Venus is so hot near its surface is not really because it is closer to the sun.

Venus probably had water and oceans for the first few hundred million years when the sun was almost a third less luminous than it is today. As our star got brighter and hotter, the water evaporated. The intense ultraviolet radiation then broke down the water vapour into oxygen and hydrogen.

Eventually, the oxygen would have combined with the Venusian crust, while the solar wind helped the much lighter hydrogen escape into space. Without oceans to convert the carbon dioxide from volcanic eruptions into limestone – as happened on Earth – the greenhouse gas remained in the atmosphere, setting off runaway global warming.

It's a worrying example of what could happen to a planet. "The same process that is warming the Earth today is what has warmed Venus to extreme temperatures," Dr Glaze explains.

Venus in profile

What's it like on the bright orb between Mercury and Earth that circles the sun at an average distance of 108 million kilometres?

High-resolution images of Venus, taken most recently by the Venus Express probe that ended its mission last year, reveal rolling, rocky volcanic plains that cover more than three-quarters of its surface, which bakes under a dull orange sky.

The rest is highland – raised "continental" plateaux and a towering mountain massif – pushed up by movements in Venus' outermost layer of crust. That's why, geologically​ speaking, the planet has a relatively young and unwrinkled face that is probably less than 500 million years old.

"Crustal resurfacing is a hypothesis that has been put forward to understand the rarity of craters on the Venusian surface," Dr Liffman says. "No one has been able to physically date the Venusian crust as we have no samples - so we don't know for sure."

But, he explains, crustal resurfacing on an approximate time scale of about 500 million years seems consistent with current observations.

Volcanic plains suggest volcanoes, of which there are millions. Previous spacecraft visiting Venus found them distributed more or less randomly around the planet.

The only volcanoes known to be active in the solar system occur on Earth and Io, the innermost of Jupiter's larger moons.

Whether or not the Venusian volcanoes are still erupting lava or molten rock is less certain. Images would need to be compared over long intervals to ascertain whether some volcanoes were active, says geoscientist Marion Anderson of Monash University.

"The assumption that heat signatures indicate active volcanism is reasonable and, until we get further evidence, it seems likely that volcanoes may still be active on Venus," she says. Variations in atmospheric compositions for years have also been interpreted as active volcanism.

Maat Mons, one of Venus' biggest volcanoes, is about nine kilometres high – about the height of Earth's Mount Everest. Lava flows at its base extend for hundreds of kilometres across the surrounding plains.

It's a shield volcano, built up by repeated eruptions. Usually assembled from layers of runny lava, shield volcanoes slope gently, such as Mauna Kea in Hawaii. Their lava is too mushy to build up into pointy-shaped volcanoes such as Mt Vesuvius in Italy or Mt Ararat in Turkey.

Despite having volcanoes, scientists believe it's unlikely that Venus has continents that move about, as they do on Earth. Ours is the only planet with a rigid outer layer of crust that is fragmented into huge plates.

"Heat from the Earth's core, and the movement of rocks within the mantle, moves these plates about – allowing continents to drift around over millions of years," Ms Anderson explains. The process is called plate tectonics.

Any planet or moon with an internal heat source can have volcanoes, she notes. "This heat can be from deep within the planet or, as is the case with some of the moons of Jupiter and Saturn, it can come from the tugging of their gas giant's gravity."

The heat that builds up must escape somehow. "One way is through volcanic hot spots that stay in the same place for millions of years. This generally gives rise to shield volcanoes, such as the ones on Venus," Ms Anderson explains. Earthquakes may also arise.

While there are currently more than 20 active hot spots on Earth, she says, Venus might have no more than one or two.

What it lacks in hot spots, Venus makes up for in impact craters, about 900 of which have been found so far. Ranging from almost two kilometres to about 280 kilometres across, some of them might have been damaged by volcanic lava or the cracking and other motions of the planet's crust.

Yet many ancient craters are still intact – despite the bouts of volcanism that resurfaced Venus many moons ago.

Previous missions

Scientists have learnt much about Venus from the 20 or so probes that have visited over the years. The first craft, NASA's Mariner 2, arrived in December 1962.

Several subsequent probes have landed. In the early 1970s, the then Soviet Union's Veneras​ 7 and 8 were the first landers to send back data from the surface. Others have transmitted pictures and analysed soil samples.

Craft that landed on Venus worked for a short time only. Venera 13 lasted the longest: Earth received its radio signals for 127 minutes, until the craft's electronic systems melted.

Measurements made by Venera 13 and its predecessors revealed that Venus' gaseous envelope consists almost entirely of carbon dioxide – a cause of global warming on Earth. The gas traps the sun's heat like a greenhouse, raising temperatures to an average of 450 degrees – and more. That is hot enough to melt lead.

Visits by spacecraft helped establish how the Venusian greenhouse began. Water once existed there, Ms Anderson says. Then, long ago, it started to evaporate, which led to a greenhouse effect that eventually cooked the carbon dioxide out of the rocks.

Radar

Despite the dense clouds – which above the equator are driven by ferocious winds of more than 100 metres per second – scientists can still get a glimpse, albeit remotely, of the surface. This is due largely to radar astronomy, which uses special equipment aboard spacecraft to spy through the clouds.

Radar, which stands for radio detection and ranging, uses a powerful radio transmitter to direct high-frequency pulses at an object and then analyses the echoes reflected back to a receiver. Using this, airport radar sweeps, for example, can locate aircraft through cloud and fog.

In addition to measuring the time signals take to bounce back, radar astronomy analyses the echoes' wavelengths in order to map the surfaces of planets such as Venus.

The high-resolution images obtained by Venus Express reveal streaks and blotches of iridescent blue, red, green and brown. These are not real but false-colour images, created to highlight the shapes of plains, volcanoes and craters. The real colour would probably be a rather drab, chocolate brown.

Infrared

Some interplanetary spacecraft are equipped with an instrument called a spectrophotometer that uses special filters to study the sunlight reflected off a planet's surface at a range of wavelengths, from infrared to ultraviolet. These are absorbed and reflected in different ways by various chemicals, revealing the composition of substances on the surface.

A spectrophotometer would not reveal much about the surface of Venus, however, because radiation from the planet's hot surface – which is mostly infrared – cannot penetrate its atmosphere.

Infrared would be absorbed in the Venusian atmosphere by carbon dioxide, but infrared detectors could study regions of the atmosphere - which is where scientists would like to concentrate their efforts.

Fact file

* As with Mercury, Venus has no moon.

* A day on Venus is longer than its year. This is because the planet spins on its axis far slower than it orbits the sun.

* Venus shows phases similar to that of Earth's sole natural satellite, the Moon.

* Due to the intense greenhouse effect on Venus, temperatures there are even higher than during daytime on Mercury — even though it's almost twice as far from the sun.

* Venus has nearly 100 times Earth's atmospheric pressure. That's the sort of colossal compression experienced at a depth of one kilometre or so beneath our oceans.

* The impact craters on Venus are not unlike the ones on heavily pockmarked Mercury, the solar system's innermost orb, on average 57.9 million kilometres from the sun.